With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, there is an increasing demand for secondary batteries or batteries having a small size, a light weight and a high energy density as a power source for driving these electronic devices. Also, in consideration of global environments, electric cars and hybrid cars have been recently developed and put into practice, so that there is an increasing demand for lithium ion secondary batteries for large size applications having excellent storage characteristics. Under these circumstances, the lithium ion secondary batteries having advantages such as a large charge/discharge capacity and good storage characteristics have been noticed.
Hitherto, as positive electrode active substances useful for high energy-type lithium ion secondary batteries exhibiting a 4 V-grade voltage, there are generally known LiMn2O4 having a spinel structure, LiMnO2 having a zigzag layer structure, LiCoO2 and LiNiO2 having a layer rock-salt structure, or the like. Among the secondary batteries using these active substances, lithium ion secondary batteries using LiNiO2 have been noticed because of a large charge/discharge capacity thereof. However, this material tends to be deteriorated in thermal stability upon charging and charge/discharge cycle durability, and, therefore, it has been required to further improve properties thereof.
Specifically, when lithium is released from LiNiO2, the crystal structure of LiNiO2 suffers from Jahn-Teller distortion since Ni3+ is converted into Ni4+. When the amount of Li released reaches 0.45, the crystal structure of such a lithium-released region of LiNiO2 is transformed from hexagonal system into monoclinic system, and a further release of lithium therefrom causes transformation of the crystal structure from monoclinic system into hexagonal system. Therefore, when the charge/discharge reaction is repeated, the crystal structure of LiNiO2 tends to become unstable, so that the resulting secondary battery tends to be deteriorated in cycle characteristics or suffer from occurrence of undesired reaction between LiNiO2 and an electrolyte solution owing to release of oxygen therefrom, resulting in deterioration in thermal stability and storage characteristics of the cell. To solve these problems, various studies have been made on materials to which Co and Al to are added by substituting a part of Ni in LiNiO2 therewith. However, these materials have still failed to solve the above-described problems. Therefore, it has still been required to provide an Li—Ni-based composite oxide having a higher crystallinity.
Further, in the process for producing the Li—Ni-based composite oxide, in order to obtain the Li—Ni-based composite oxide having a high packing property and a stable crystal structure, it is required to use Ni composite hydroxide particles which are well controlled in properties, crystallinity and contents of impurities, and calcine the particles under the condition which is free from inclusion of Ni2+ into Li sites thereof.
More specifically, it is required to provide Li—Ni-based composite oxide capable of exhibiting a high packing property, a stable crystal structure and an excellent thermal stability upon charging as a positive electrode active substance for a non-aqueous electrolyte secondary battery.
In order to improve the above thermal stability, it is important to suppress a reaction between oxygen released from the composite oxide and an electrolyte solution. Although the Li—Ni-based composite oxide produces Ni4+ in a charged condition of the cell, the Ni ion in such an oxidized state is very unstable. As a result, oxygen tends to be readily released from the composite oxide, so that Ni4+ tends to be reduced into Ni+ or Ni2+. For this reason, when using the composite oxide as a positive electrode active substance for the lithium secondary battery, problems concerning a thermal stability of the secondary battery such as generation of heat and firing tend to be caused in a charged condition of the cell.
In addition, it is considered that the problems concerning a thermal stability of the lithium secondary battery, in particular, generation of heat and firing, are caused at a solid-liquid boundary between the electrode active substance and the electrolyte solution as a starting point.
In view of these facts, in order to improve a thermal stability of the Li—Ni-based composite oxide, it is considered to be effective that the contact surface thereof with the electrolyte solution, i.e., the surface of the respective particles, is coated with other stable elements.
Hitherto, in order to improve various properties of LiNiO2 particles such as stabilization of a crystal structure and charge/discharge cycle characteristics, various improving methods have been attempted. For example, it is known that the surface of the respective Li—Ni-based oxide particles is coated with a compound such as lithium manganate (Patent Documents 1 to 5).
The safety of lithium ion cells becomes more and more important. For this reason, the studies on surface modification of active substance particles by noticing an interfacial reaction thereof during charging and discharging of the cells have been made as to various materials. For example, there are known many techniques using an Li—Co-based composite oxide as a core material (Japanese Patent Application Laid-Open (KOKAI) Nos. 2006-331939 and 2007-18743) or using an Li—Mn-based spinel oxide as a core material (Japanese Patent Application Laid-Open (KOKAI) No. 11-71114(1999)). However, in any of these conventional techniques, from the viewpoint of a thermal stability, the core materials used therein are inherently stable. Therefore, the surface modification of the core particles mainly aims at preventing deterioration in cycle characteristics of the cells.
In the present invention, an Li—Ni-based composite oxide is used as the core material, and an object of the present invention is not to improve the cycle characteristics, but to solve the problems concerning a thermal stability peculiar to the Li—Ni-based composite oxide.
Patent Document 1: Japanese Patent Application Laid-open (KOKAI) No. 7-235292
Patent Document 2: Japanese Patent Application Laid-open (KOKAI) No. 9-265985
Patent Document 3: Japanese Patent Application Laid-open (KOKAI) No. 10-236826
Patent Document 4: Japanese Patent Application Laid-open (KOKAI) No. 11-67209
Patent Document 5: Japanese Patent Application Laid-open (KOKAI) No. 2007-213866